This invention relates to a valve control system for internal combustion engines which are capable of changing the valve lift characteristic of inlet and/or exhaust valves, and more particularly to a valve control system of this kind for automotive vehicles equipped with driving wheel slip control systems.
There is shown in FIG. 10 the general relationship between the engine rotational speed and the engine output torque in an internal combustion engine which is capable of switching the valve lift characteristic of inlet and/or exhaust valves between a low speed valve lift characteristic suitable for a lower engine rotational speed region and a high speed valve lift characteristic suitable for a higher engine rotational speed region. (Throughout the specification and claims, the term "valve lift characteristic" represents a valve opening period of an inlet and/or exhaust valve and/or an amount of lift of the inlet and/or exhaust valve.) In the lower engine rotational speed region, larger engine output torque can be obtained when the low speed valve lift characteristic is selected, while in the higher engine rotational speed region, larger engine output torque can be obtained when the high speed valve lift characterisitic is selected. Normally, the valve lift characteristic which enables to obtain larger engine output torque is selected depending upon engine operating conditions (mainly upon the engine rotational speed). Therefore, if the valve lift characteristic is changed to one which is opposite to a normally-selected valve lift characteristic, the engine output torque can be decreased.
This feature of selection of the valve lift characteristic is utilized, e.g. in Japanese Patent Application No. 63-330938 filed by the present assignee. This application proposes a valve control system in which the degree of slip of driving wheels is reduced by changing the valve lift characteristic to one which enables to obtain smaller engine output torque when an excessive slip state of the driving wheels is detected.
The above proposed valve control system is effective in suppressing excessive slip of the driving wheels, since the engine output can be effectively decreased by the changeover of the valve lift characteristic. However, there is a possibility that on a road surface having a relatively low friction coefficient, i.e. on a slippery road surface, the changeover of the valve lift characteristic is frequently carried out, which can shorten the life of the valve lift characteristic-switching mechanism.
Further, when the valve lift characteristic is changed over, the engine output torque can be drastically changed. Particularly, when the valve lift characteristic is restored to a normal one which enables to obtain larger engine output torque after suppressing excessive slip of driving wheels, there is a great possibility that the engine output torque increases to bring about an excessive slip state of the driving wheels again, which degrades the controllability (driveability) of the vehicle.
In the meanwile, it is conventionally known e.g. from Japanese Provisional Patent Publication (Kokai) No. 58-8436 to control the slip state of driving wheels by reducing the engine output through cutting off fuel supply (fuel cut) or leaning the mixture to the engine when an excessive slip state of the driving wheels is detected.
However, in general, the engine output can be reduced more responsively by fuel cut or mixture-leaning than by changing the valve lift characteristic, which leads to the following disadvantage when the two kinds of manners for reducing the engine output are used in combination:
Although when an excessive slip state of the driving wheels is detected, the valve lift characteristic is changed to one which enables to obtain a smaller engine output, an enging output-reducing effect is obtained in a relatively short period of time due to the fuel cut or mixture-leaning to eliminate the excessive slip state of the driving wheels, so that the valve lift characteristic is restored to a normal one which enables to obtain a greater engine output in a short period of time. This restoration of the valve lift characteristic to the normal one results in an increase in the engine output, and therefore can bring about an excessive slip state again, which causes changeover of the valve lift characteristic again. Thus, hunting of the valve lift characteristic can occur, particularly when the vehicle continues to run on a road surface which is low in friction coefficient, i.e. on a slippery road surface. This degrades the controllability (driveability) of the vehicle and can even shorten the life of the valve lift characteristic-changing mechanism.
Further, according to the proposed valve control system, when the valve lift characteristic is changed over, the engine output can drastically change. If the engine output drastically changes on a road surface having a low friction coefficient, which is liable to bring about an excessive slip state of the driving wheels, there can occur a drastic change in the slip state of the driving wheels, and further a sudden change in the distribution of the wheel load. Therefore, changeover of the valve lift characteristic on such an occasion can result in degraded controllability of turning of the vehicle.
In the meanwhile, as recognized in general, a driving wheel of an automotive vehicle undergoes a slip when the vehicle is started to run or when it is accelerated, if the driving force of the driving wheel surpasses a frictional force developed between the tire of the driving wheel and the road surface [=the coefficient of friction between the tire and the road surface.times.load of the vehicle weight on the driving wheel (wheel load)]. Conventionally, a driving wheel slip control system has been proposed, e.g. by Japanese Patent Publication (Kokoku) No. 51-48334, in which the degree of the slip is detected from a difference .DELTA.V between the driving wheel speed and the trailing wheel speed, and a feedback control amount is calculated based on the detected wheel speed difference .DELTA.V and a predetermined control gain, so that the output torque of the driving wheels is reduced in accordance with the feedback control amount. The reduction of the output torque of the driving wheels in accordance with the feedback control amount is carried out by fuel cut/leaning of the mixture, or by changing ignition timing.
If the valve lift characteristic is changed to one enabling to obtain a smaller engine output by the proposed valve control system when an excessive slip state of the driving wheels is detected, so as to eliminate the excessive slip state, the manner of change in the engine output (particularly the increase rate of the engine output) varies. More specifically, when the valve lift characteristic is changed to one enabling to obtain a smaller engine output, the engine output is once reduced upon the changeover. If the excessive slip state of the driving wheels is not eliminated by this reduction of the engine output, excessive slip control is continued by holding the valve lift characteristic at the one enabling to obtain a smaller engine output. Therefore, the engine output changes in a manner different from that in the case where a normal valve lift characteristic enabling to obtain a larger engine output is selected.
In the meanwhile, in the above conventional driving wheel slip control system, the control gain for calculating the feedback control amount is set to a valve suitable for eliminating an excessive slip state of the driving wheels in a short time period, by taking into consideration the time lag in the control system from the time point the excessive slip state is detected to the time point reduction of the engine output by fuel cut etc. actually takes effect while a normal valve lift characteristic is selected. However, if this feedback control is employed in combination with the above-mentioned valve lift characteristic-changing control to reduce the engine output for elimination of an excessive slip state of the driving wheels, the manner of change in the engine output is varied by changeover of the valve lift characteristic to the one enabling to obtain a smaller engine output carried out when the excessive slip state is detected, whereby the above time lag varies. Consequently, if the same control gain is used for the feedback control irrespective of change in the valve lift characteristic, the excessive slip state cannot be eliminated or it can take much time to eliminate the excessive slip state due to the variation in the time lag caused by changeover of the valve lift characteristic. Therefore, in the case where the conventional driving wheel slip control system is applied to a vehicle in which is installed an engine capable of changing the valve lift characteristic, and at the same time the valve lift characteristic is changed to one enabling to obtain a smaller engine output when an excessive slip state of the driving wheels is detected, the excessive slip control has to be improved, especially so as to compensate for the varied increase rate of the engine output caused by changeover of the valve lift characteristic.